Assessment of renal acidification capacity of stone formers by acid challenge test

Acid challenge test was carried out on 87 subjects (37 normal and 50 radiologically proven stone formers) to assess their renal acidification capacity. NS were given three doses viz. 75, 100 and 150 mg NH₄Cl/Kg body weight while the SF were tested with only 150 mg dose which was found to be an adequate dose. Ammonium chloride was given in gelatin capsules with breakfast and hourly urine samples were collected for next 7 hours. pH, creatinine, calcium, inorganic phosphorus, citrate, titrable acid and ammonium were analysed in all the samples. The incidence of renal tubular acidosis (RTA) in NS, tested with 75 mg and 100 mg/kg doses of ammonium chloride, was 50% and 10% respectively while it was nil and 24% respectively in NS and SF tested with 150mg/kg dose. Complete RTA was detected in one SF. No significant difference was observed in urinary profile of NS and SF except that the hourly titrable acid excretion was lower in later. A comparison between with and without RTA SF revealed that titrable acid and phosphate excretion decreased in RTA defect. Overall data indicate that (a) RTA was an etiologic factor in one fourth of the SF; (b) an oral acid challenge test is a good adjunct to detect this condition and (c) acid challenge increases calcium, phosphate and titrable acid excretion and decreases citrate.